New Surface Aspects towards Photocatalytic Activity of Doped Supported Titanium Dioxide

The present work aims to synthesize nanoscale well dispersed TiO 2 /SiO 2 and TiO 2 /Al 2 O 3 nanoparticle photocatalysts via an impregnation method for the removal of methyl orange, which was used as a model compound of organic pollutant in wastewater, from an aqueous medium. Also within this frame work, La and Ce metals were loaded onto the surfaces of TiO 2 /SiO 2 and TiO 2 /Al 2 O 3 by an impregnation method to enhance the photocatalytic activity of the nanoparticles; the activities and physicochemical properties of the photocatalysts were compared before and after loading of metallic La and Ce. The oxide system was characterized by different techniques, including XRD, UV-Vis spectroscopy, FT-IR spectroscopy, SEM, and EDX spectroscopy. Finally, the optimal conditions to complete the photocatalytic oxidation ofmethyl orange dyewere studied.Thiswork holds promise for the efficient photodegradation of pollutants by nanoparticle photocatalysts.


Introduction
Methyl orange is widely used as an acid-base indicator, and it has also been employed in chemical, technological, and biomedical industries. Its chemical properties have been widely investigated in aqueous solutions and in water and organic mixed solvents [1]. Additionally, this compound has been widely used in printing, paper, food, pharmaceutical, and textile industries, as well as research laboratories [2]. It is an orange-colored, anionic dye that possesses absorbance maximum at 464 nm. Methyl orange is a compound that exists in textile wastewaters and has various harmful effects on humans. For example, it may cause eye or skin irritation, and inhalation may cause gastrointestinal irritation with nausea, vomiting, and diarrhea. There are various technological processes such as adsorption, biodegradation, chemical methods, electrocoagulation, electrochemical reduction and oxidation, indirect electrooxidation with strong oxidants, and photocatalytic degradation that can be used for the treatment of wastewaters, including those containing synthetic dyes [3,4]. A number of studies have dealt with the heterogeneous photocatalytic decomposition of many types of azo dyes by UV, visible light, and solar irradiation. The dangerous accumulation of dyes in the environment can be avoided by developing effective ways for their removal either by the degradation of such organic pollutants to less harmful compounds [5] or by their complete mineralization [6], such as titanium dioxide, which has been widely used as a photocatalyst for the degradation of MO. The photocatalytic efficiency of a nanocrystalline TiO 2 system is strongly influenced by light sources and the recombination of photoinduced electrons and holes. It is known that gold deposited onto TiO 2 surface inhibits the recombination of photogenerated electrons and holes. The photocatalytic deposition of metals onto titanium dioxide films is one of the most interesting methods for producing titanium dioxide/metal composites. The pairing of TiO 2 with other metals, including Au, Ti, Ce, Fe, and Pt [6], has been used to increase the activity of TiO 2 towards the degradation of azo dyes, such as methyl orange.

Preparation of TiO 2 /Al 2 O 3 .
Five grams of aluminum oxide (Al 2 O 3 ) was mixed with a certain amount of titanium isopropoxide (TIP) or titanium tetra chloride (TiCl 4 ). Then, a minimal amount of distilled water was added dropwise to the mixture, under vigorous stirring, to obtain 2.5, 5, 10, and 20 wt.% TiO 2 , as shown in Table 2. The formed paste was completely mixed to obtain a homogeneous mixture, which was then dried in an oven at 100 ∘ C for 24 h. The powder was ground using a mortar and pestle, followed by calcination at 550 ∘ C for 4 h.

Preparation of La Metal Loaded TiO 2 /Al 2 O 3 .
Lanthanum(III) nitrate hexahydrate, La(NO 3 ) 3 ⋅6H 2 O, was used as the source of the La(III) dopant. The detailed procedure for the preparation of 4 dopant concentrations (0.05, 0.1, 0.2, and 0.3 mole ratio of La to Ti) by the impregnation method is as follows. An appropriate amount of La(NO 3 ) 3 ⋅6H 2 O was dissolved in a small amount of distilled water. This solution was then added dropwise to 5 g of TiO 2 /Al 2 O 3 (5 wt.% TiO 2 ) with constant stirring, as shown in Table 3. The product was dried in an oven at 100 ∘ C for 24 h and was ground to a fine powder. Then, it was calcined at 550 ∘ C for 4 h in a muffle furnace. 6 , was used as the source of the Ce(IV) dopant. The detailed procedure for the preparation of 4 dopant concentrations (0.05, 0.1, 0.2, and 0.3 mole ratio of Ce to Ti) by the impregnation method is as follows. The appropriate amount of (NH 4 ) 2 Ce(NO 3 ) 6 was dissolved in a small amount of distilled water. This was then added dropwise to 5 g of TiO 2 /Al 2 O 3 (5 wt.% TiO 2 ) with constant stirring, as shown in Table 4. The product was dried in an oven at 100 ∘ C for 24 h and was ground to fine powder. Then, it was calcined at 550 ∘ C for 4 h in a muffle furnace.

Characterization Techniques and Control
Test. The synthesized samples were evaluated and characterized by determining photocatalytic activity, particle size, and phase using XRD, UV-Vis, FT-IR, SEM, and EDX. Experimental details are as follows.

X-Ray Powder Diffraction (XRD) Analysis.
The crystalline phase and crystallite size of all catalyst nanoparticle samples were analyzed by X-ray powder diffraction (XRD) using a Rigaku X-ray diffractometer system equipped with a RINT 2000 wide angle Goniometer and Cu K radiation ( = 0.15478 nm) at a power of 40 kV × 30 mA. The intensity data were collected at room temperature over a range of 2 from 10 to 80 ∘ .

Ultraviolet-Visible (UV-Vis) Spectroscopy.
The concentration of the dye was followed up using UV-Vis spectrophotometer (Thermo Fisher Scientific Evolution 300).

Fourier Transform Infrared (FT-IR) Spectroscopy.
To study the surface chemical structures of all catalyst nanoparticle samples, Fourier transform infrared (FT-IR) spectra were recorded on a PerkinElmer FT-IR instrument 100 series.

Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray (EDX) Spectroscopy. Surface morphologies and
shapes of all catalyst nanoparticle samples were examined via a field-emission scanning electron microscope (SEM), which was obtained using JEOL JSM-7600F. This system was combined with energy dispersive X-ray spectroscopy for composition and elemental analysis.

The Experiment for Photocatalytic Degradation of Methyl
Orange. The photocatalytic activity of each prepared sample was evaluated by degradation of a pollutant in water. Methyl orange (MO), a common textile dye, was used as the model pollutant. The activity of the photocatalyst was tested by analyzing the decomposition of MO under UV irradiation. The MO solutions varied in concentration from 1 to 20 ppm and were prepared by dissolving MO powder in distilled water: 250 mL of 1 ppm MO solution was added to a Pyrex beaker (600 mL) and 0.5 g of TiO 2 /SiO 2 , The suspension was magnetically stirred in the dark for various time intervals. An 8 W Philip's lamp was used as the UV source at a fixed distance of 25 cm from the top of the magnetic stir bar. All components of the experimental setup were located inside a box, as shown in Figure 1. A 10 mL sample was taken from the beaker at various time intervals, and the photocatalyst was separated from the solution by filtration. Absorbance measurements were also recorded in the range of 200-600 nm using UV-Vis spectrophotometer.
The concentration of MO solutions was determined by measuring the absorbance at approximately 465 nm using UV-Vis spectrophotometer (Thermo Fisher Scientific Evolution 300, USA).

Characterization of Investigated Samples.
For catalyst characterization, knowing the catalyst's texture and phase, as well as its chemical composition, represents an essential minimum. XRD, FT-IR, SEM, EDX, and UV-Vis are basic methods that provide the required information. In Figure 6, it can be observed that the lower absorption bands (326 and 418 cm −1 ), which are assigned to the M-O vibration (M = Ti, La, or Al) [9][10][11][12], are well separated in undoped samples; however, even with a doping level of 0.05 wt.%, only unresolved bands are observed.        Table 5 provides the experimental EDX analysis of some selected samples compared to the theoretical wt.% of same samples.

EDX Analysis of Samples.
Tables 5-7 present the data analysis for all investigated samples obtained by calculating the theoretical and the actual wt.% loading, in addition to determining the excess 6 International Journal of Photoenergy where (wt.%) 1 is the theoretical weight percentage of loading and (wt.%) 2 is the surface weight percentage of the metal oxide measured by EDX analysis [13]. From these tables it could be observed that the presence of La 2 O 3 , as a dopant, facilitated the diffusion of TiO 2 into the alumina support, as reflected by the remarkably small amount  of TiO 2 when compared to values for the theoretical load and those observed for undoped samples. The facile diffusion of TiO 2 into alumina appeared to exist even in undoped samples. It is suggested that approximately half of TiO 2 diffused into undoped samples when compared to the theoretical loading values. The introduction of La 2 O 3 as a dopant with a 0.05 Ti : La mole ratio resulted in a dramatic increase (30%) of TiO 2 diffusion. From the data below, we can conclude that there are two types of active sites: one is predominantly a surface site associated with TiO 2 , and the other is bulk or diffused TiO 2 . From Figure 16, which indicates the catalytic activity of La-doped and undoped samples, we can observe that the sample with a 0.05 Ti : La mole ratio showed the highest catalytic activity of all samples. Increasing the amount of the dopant, however, did not increase the catalytic activity but decreased it when compared to the undoped samples. This behavior could be explained when we examined the EDX data. This analysis showed that the 0.3 Ti : La mole ratio sample reduced the amount of diffused TiO 2 by approximately 30% when compared to the 0.05 sample. The direct conclusion based on the above behavior is that the diffused TiO 2 catalytic active site is much higher than that of surface one. The decrease in diffusion of TiO 2 observed upon increasing the amount of dopant may be understood as a competition between the affinity of TiO 2 for La 2 O 3 and that for alumina. This competition could be further influenced by the intermolecular interactions between molecules of La 2 O 3 , which could be favored at high levels of dopant (0.3 mole ratio). This possible interaction was not observed with XRD, due to the detection limit; however, it is well observed from FT-IR spectra in Figure 6. In addition, we can discuss the observation of M-O bands in the FT-IR spectra of doped samples (Figure 6), which could also result from the dopant facilitating the diffusion of TiO 2 into alumina. Hence, the presence of multiple metal-oxygen vibrational bands in the same vicinity could be evidence of a possible interaction between them.
Returning to the CeO 2 -doped samples, the EDX analysis showed nearly the same enhancement of TiO 2 diffusion at the low doping concentration (0.05 mole ratio), which was not reflected in its catalytic activity. Moreover, by increasing the dopant level (0.3 mole ratio) and through a different spot analysis, CeO 2 seemed to aggregate on the surface. This could be explained as the affinity of CeO 2 towards alumina being higher than its affinity for TiO 2 , which is in contrast to La 2 O 3 , which first interacts with TiO 2 . Figures 12-15 showed the photocatalytic activity for differently loaded samples of TiO 2 /Al 2 O 3 and TiO 2 /SiO 2 . From these curves, we could see that the titanium isopropoxide precursor produced TiO 2 evenly dispersed across the surfaces of either Al 2 O 3 or SiO 2 and yielded samples with high photocatalytic activity. It was also observed that the photocatalytic activity of Al 2 O 3 support was much higher than that of SiO 2 .

Effect of Precursors and Support Type on Catalytic Activity.
For Al 2 O 3 support it could be concluded that the 5 wt.% loading was the optimum loading with respect to catalytic activity and the decomposition rate of the organic dye. Figure 16   it could be observed that as doping decreases, the catalytic activity increases. Photocatalysis reaches a maximum at a doping of 0.05 mole ratio La, which is greater than the activity of the undoped sample. Moreover, Figure 17 showed the effect of different doping levels using Ce at the same concentrations. The same trend could be observed for Ce-doped samples as was noted for La-doped samples; however, the doping with Ce leads to a remarkable decrease in catalytic activity compared with the undoped samples.

Conclusions
From the above research, the following conclusions were drawn: (1) Titania supported on silica or alumina could be used as a good photocatalyst for the degradation of organic dyes.
(3) The optimum loading value was found to be 5 wt.% TiO 2 which insures its presence in nanoscale.
(4) The alumina support (corundum phase) was found to be more effective than that of amorphous silica.
(5) In trying to increase catalytic activity by dopant inclusion of either La 2 O 3 or CeO 2 , we determined that La 2 O 3 enhances the catalytic activity more than CeO 2 .
(6) The interactions between La 2 O 3 , TiO 2 , and alumina were correlated with catalytic activity.
(7) The 0.05 mole ratio for the dopant was found to be the optimum value, above which the catalytic activity decreased.